Production Parts out of Sheet Steel

G-Auto · Feb 9, 2007

I have 2000 parts I need to make out of 1/8" thk steel. The parts are small, about 1.5"x1.0" and are heat treated after cutting. What's the most efficient way to wire EDM sheet steel parts like this?

One option is to heat treat alloy steel rounds about 3" long then cut the part shape then baloney cut the parts.

Another way would be to stack 12"x6" sheets about 2.5" tall, compress the sheets in a vise then tig weld the stack together at several locations around the outside. Parts would be cut from the stack of sheets, sent out for HT then kissed on the surface grinder to true them up. I think this way is better than the baloney cut parts.

I am new at this with a recently purchased Mits H110 so any help is appreciated.

TIA

Hollistergc · Feb 9, 2007

I would stack the stock and cut the parts (your option 2). However, I would heat treat and grind the material before cutting unless there is subsequent operations that require the material to be cut soft.

roysol · Feb 9, 2007

I second the stacking. I do parts like this all the time, it works great!

I have welded stacks, and also sandwiched them in between a top and bottom plate that's drilled and tapped.

Think about how you're going to hold the stack, if you use sandwich plates, you can make them ovesize so they can be clamped to the table. If you have an EDM style vise, you can grind the sides of the welded stack to true them up.

12x6x2.5 may be a bit too big. The sweet spot for max speed on most machines is between 1 and 2 inches thick, after that you get diminishing returns.

Also, 6x12 may end up with too much bow in the center of the stack, since you can only restrain the edges. I'd be tempted to try a layout 1/4 that size.

Cheers!

Roy Solomon

G-Auto · Feb 10, 2007

Gents, Thanks for the replys! Roy I like your sandwich idea. Perhaps I could make the bottom plate thicker (1/2") and have one long side extend so it can be bolted to the table and the sheets extend over the cutting area.

G-Auto · Feb 10, 2007

Hollistergc, I think in the first heat treat run I will include some of the plates and try heat treating first. I am just concerned about the plates warping too much so grinding can't clean them up. Thanks for the advice.

MitsTech · Feb 10, 2007

Why not heat treat them before WEDM? Then you can grind the plates flat prior to any EDM work being done. In fact, they will probably EDM faster in the hardened state than they will soft.

SeymourDumore · Feb 10, 2007

I'd say when you grind them is dictated more by the part and the dimensions, but as the others said, laminate them no more than 2" thick. Put the whole thing between a 1/2" or thicker plates and heat treat. They WILL remain or get straight. EDM the shape and done.
As for TIG, that depends on the material. If you can penetrate deep enough you're fine, but you have to consider that during the time it's in the furnace the TIG weld may let go here and there, and there is not many things worse than 1 of 20 welds letting go and flappin' in the flushing.
I TIG for the exotics like Inco, but MIG for everything else.

RJT · Feb 10, 2007

We heat treat and grind first, use sandwich plates screwed together, leave cutoff tabs to run unattended at night (assuming you have a wire threader), then cut off tabs when you have an operator. RJT

G-Auto · Feb 10, 2007

RJT,

How big of a cutoff tab do you use and how do you cut it off? By hand?

My H110 does not have a threader so I am thinking of lacing all the parts together in one big burn....not roughing everything at once then finishing but roughing a part then going back for the finish skims then proceding for the next part. Will need to see if this will work or not.

MitsTech · Feb 11, 2007

.100" is plenty for a holding tab on parts that size.

Rough around to the tab, then reverse direction and do your first skim. Go back and forth like this until all of your rough/skims are done. At the end of your last skim, when the wire is next to the holding tab, put an M01 in your program. You don't want to cut off the slugs if you're not there to stop the machine the moment it cuts off, you could drop a slug and cause a collision with the lower head.

Use a startup E-pack and roughing offset, turn on fluid/wire/machining and cut off the tab.

This will leave you with a small tab to grind off, but it's an unfortionate reality of EDM.

There are ways to hold punches in so the holding tab can be roughed/skimmed such as magnets, glues, shims, etc, but that won't work on stacked parts.

SeymourDumore · Feb 12, 2007

G-auto

What accuracy do you need? Do you really need skim cuts?

G-Auto · Feb 12, 2007

SeymourDumore

I have three loacations on this part where I need +/- 0.0005" and a 32 finish. The rest can be +/- 0.003".

I am getting the machine delivered this coming Saturday (as long as the weather holds out here in eastern PA). I am not sure if this machine will hold 0.0005" on one pass. The test cut I saw the machine do in 1/2" A2 was off by 0.0013" (big) with just one cut. I figured with a skim it might come closer to the correct size.

Also still need to figure out programming. the fellow I am purchasing it from used to work for Mits and he is going to help me get going. I have been reading the manuals. They are interesting to say the least.

SeymourDumore · Feb 12, 2007

Location or feature tolerance?
Feature, +/-.0005 is close but usually doable with rough only. Position though, better be less than that first-off.
If it's position, is it MMC or RFS?

G-Auto · Feb 12, 2007

The tolerances on the part I am doing are location not feature like flatness. There is no GDT spec to teh dimensions/tol. Critical areas are two flats that are parallel to each other and .050 and .080 away from a hole.

If I could cut this part in one shot that would be really good.

In your experience if you wire a cube 1" by 1" out of a stack 2" high can typical wire machines hold +/-0.0005" on the 1" dimension on one cut only?

SeymourDumore · Feb 12, 2007

Yes. After about 2 or 3 stacks you should be able to dial in the settings to find the sweet spot.
Yes, you'll find that the bottom parts will be slightly larger, but the difference from top to bottom should be under .0003, so dial-in is possible.

MitsTech · Feb 13, 2007

Bumping up the wire speed a notch or two can help with taper on tall parts, albeit at the expense of using slightly more wire. Still faster and uses less wire than taking a skim cut.

Are you planning on signing up for a Technical Service contract? You will need one if you ever have to call for phone support. PM me if you have any questions on pricing or coverage.

roysol · Feb 13, 2007

Everybody's getting you on the right track, but they may have overlooked the 32 RMS finish spec. You will be 3 times that on a rough cut, and would have to take at least one skim, if not two to achieve that.

I have found on parts that "might" be ok with out trimming, if you want to be sure, and have good consistency, even one trim helps alot.

Also, keep in mind that the parts will move some as stress is relieved, and if you need a hole accurately in relation to another feature of the part, some care must be taken.

Cheers!

Roy Solomon

G-Auto · Feb 13, 2007

I do need one hole accuratly located Roy.

My plan is to first drill a 3/32" hole in the plates before mounting. (I don't have a hole popper) The first step in edm will be to open the hole up to 5/32" then cut the shape so the hole ends up in the correct position.

Like MitsTech said there will be a cut off tab but I can live with this. There are some areas that are not critical on the part. Ill leave as much as I can on the tabs (like about 1/4") and cut the critical features. Then I can cut the tab off and leave about .010 to break off.

Thnaks for all the help and advice.......

SeymourDumore · Feb 13, 2007

Now we're getting into the nitty-gritty part.
Does the whole thing needs 32 finish, or only the hole? If I was to do that, the hole would be the very last thing to be finished. Edm to within .002, hone after all else is done. Size and RMS in one shot.
Also, not too sure about how you guys check RMS of parts, but I often get much better than that at rough only. Checked with the surface-prick instrument and not by the prick-inspector.

How accurate is the hole location? How is it given, and how is it measured?

MitsTech · Feb 13, 2007

Checked with the surface-prick instrument and not by the prick-inspector.

You really had me laughing on that one!

If the hole diameter is only 5/32, it won't take long at all to do a couple of skim cuts.

G-Auto, if you can get the hole size within about .015" or .020" and on location by pre-drilling, you will save a bunch of time and eliminate potential wire break headaches by not having any slug at all in the hole. Tiny little thin slivers bouncing around in cut will short the cut and break wire constantly. You may have to turn down flushing slightly to prevent "belly" but it shouldn't slow you down that much, I think the tradeoff in stability and not having to drop a pile of tiny slugs would be worth it.

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